Doug McClelland, age 12, of St. Catharines, Ontario, Canada, for his question:
How come only some bodies of water are salty?
Lakes and rivers, streams and creeks are filled with what we call fresh water. The seas and oceans are filled with bitter salty water. However, Utah's wonderful Great Salt Lake is several times saltier than the salty sea and there also are other salt lakes in the world. This is somewhat mysterious. It means that whatever adds salt to the seas also selects certain bodies of inland water to get a helping of salty chemicals.
We live in a watery world and geologists suspect that the basic supplies deluged down when the earth was young. This first water from the massive clouds was unsalted. Then, through the ages, a complicated process went to work.
The process involved the sun, the hovering clouds and the rocks of the earth's crust. This team quickly set in motion the revolving water cycle which never stops. On a global scale, every day the beaming sun evaporates some 250 cubic miles of surface water into the air. After breezy rides as gaseous water vapor and misty clouds another 250 cubic miles of moisture falls back to the surface as rain and snow.
Meantime, much of the daily downpour gathers in rivers that run off to empty their contents into the seas. However, streams of moving water tend to dissolve mineral chemicals as they brush by the rocks. These invisibly small particles are carried along and dumped with the river water into the seas. The tiny traces of salty chemicals in river water are not noticeable. But for billions of years the rainy runoff has been leaching these traces from the land and donating them to the ocean.
When the sun evaporates surface moisture, the dissolved chemicals are left behind. This explains how the enormous oceans got to be salty. But what about the smaller bodies of salty lake water?
They are exposed to the same process that makes the sea salty, only on a smaller scale. A freshwater lake is filled and drained by moving streams or so called freshwater. A saltwater lake has no stream to drain its water back to the sea. As a rule, it is fed by streams and perhaps by springs that well up in its floor. Meantime, the sun evaporates moisture from its surface.
The streams that fill such a lake bring in the usual traces of salty chemicals. When the sun evaporates water from such a lake, these dissolved minerals remain, making the lake, like the sea, gets saltier all the time.
Sometimes a landslide cuts off an arm of the sea. This body of water is trapped as an inland lake. Naturally it becomes a lake of salt water. Chances are, it has no freshwater streams to feed it and no streams to drain it. In this case, the beaming sun gradually evaporates its moisture and once again the salt gets saltier and saltier.